Glass-filled polymer (or glass-filled plastic), is a mouldable composite material. It comprises short glass fibers in a matrix of a polymer material. It is used to manufacture a wide range of structural components by injection or compression moulding. It is an ideal glass alternative that offers design flexibility, chemical, durability, and chemical and shatter resistance. 
The first mouldable composite was Bakelite. This used wood flour fibres in phenolic resin as the thermoset polymer matrix. As the fibres were only short this material had relatively low bulk strength, but still improved surface hardness and good mouldability.
A wide range of polymers are now produced in glass-filled varieties, including polyamide (Nylon), acetal homopolymers and copolymers, polyester, polyphenylene oxide (PPO / Noryl), polycarbonate, polyethersulphone
Bulk moulding compound is a pre-mixed material of resin and fibres supplied for moulding. Some are thermoplastic or thermosetting, others are chemically cured and are mixed with a catalyst (polyester) or hardener (epoxy) before moulding.
Compared to sheet materials
Bulk glass filled materials are considered distinct from fibreglass or fibre-reinforced plastic materials. These use a substrate of fabric sheets made from long fibres, draped to shape in a mould and then impregnated with resin. They are usually moulded into shapes made of large but thin sheets. Filled materials, in contrast, are used for applications that are thicker or of varying section and not usually as large as sheet materials.